CHARACTERIZATION OF THE ANTINOCICEPTIVE AND SEDATIVE EFFECT OF AMITRAZ IN HORSES

Amitraz, an acaricide used to control ectoparasites in animals has a c omplex pharmacological activity, including az-adrenergic agonist actio n. The purpose of this research was to investigate the possible antino ciceptive and/or sedative effect of amitraz in horses. The sedative ef fect of the intravenous (i.v.) injection of dimethylformamide (DMF, 5 mL, control) or amitraz (0.05, 0.10, 0.15 mg/kg), was investigated on the head ptosis test. The participation of alpha(2)-adrenergic recepto rs in the sedative effect provoked by amitraz was studied by dosing yo himbine (0.12 mg/kg, i.v.). To measure the antinociception, xylazine h ydrochloride (1 mg/kg, i.v,, positive control) and the same doses of a mitraz and DMF were used. A focused radiant light/heat directed onto t he fetlock and withers of a horse were used as a noxious stimulus to m easure the hoof withdrawal reflex latency (HWRL) and the skin twitch r eflex latency (STRL). The three doses of amitraz used (0.05, 0.10 and 0.15 mg/kg) provoked a dose-dependent relaxation of the cervical muscl es. The experiments with amitraz and xylazine on the HWRL showed that after i.v, administration of all doses of amitraz there was a signific ant increase of HWRL up to 150 min after the injections. Additionally, there was a significant difference between control (DMF) and positive control (xylazine) values up to 30 min after drug injection. On the o ther hand, the experiments on the STRL show that after administration of amitraz at the dose of 0.15 mg/kg, a significant increase in STRL w as observed when compared with the control group. This effect lasted u p to 120 min after injection. However, no significant antinociceptive effect was observed with the 0.05 and 0.10 mg/kg doses of amitraz or a t the 1.0 mg/kg dose of xylazine.